Where to find _fft_c2c_mkl implementation?

I would like to know how _fft_c2c_mkl is implemented?

SpectralOps.cpp has only the interface but not the implementation inside, I want to know how it is generated.

Based on SpectralOps.cpp this call redirects to MKL methods such as DftiComputeForward or pocketfft depending on the CPU architecture and how PyTorch was built.
At least MKL is closed source so you won’t be able to check its source code. PocketFFT seems to be a modification of FFTPack so you might be able to find the source.

Thank you so much for your enthusiastic response, I found it and it was indeed just what I needed!